The Perfect Time to Plant Out Seedlings and Young Plants

20 May 2011, written by Jeremy Dore

Most gardeners have to learn the hard way about the best time to plant out seedlings they have raised indoors or in a greenhouse. In the first year some of their plants will be a success while others will keel over and die for no apparent reason and it can take several seasons with all their weather variations before it becomes apparent why they failed. It’s a pity because a little good advice can save a lot of disappointment and significantly increase the levels of success in the garden. With this in mind here’s my guide to deciding when pot-raised plants should leave home and settle down in your vegetable beds.

To illustrate the problem take a look at the picture below which shows two tomato plants from my own greenhouse this year. Both were started at the same time, raised in identical conditions from the same seed packet using the same potting compost. The plant on the right was transferred to the greenhouse vegetable bed at an optimal time whereas the one on the left was kept in a pot too long (I often end up raising more plants than I have room for!) In just over three weeks the difference in growth and plant health is clear. Getting plants out into the main vegetable bed is essential for good growth! However, transfer them too early and you risk them being damaged by late frosts or setting their progress back with adverse weather conditions.

Tomatoes in a pot vs tomatoes in soil

Signs That it's Time to Plant Out

Raising plants in pots or seed trays is a bit like giving them life support. Intensive care is necessary to ensure that things that are usually available naturally in the garden (light, water, nutrients) are supplied in the correct quantities. Light often needs to be supplemented to prevent ‘leggy’ growth, watering needs to be daily and even good quality seed-starting potting soil will only have limited supplies of nutrients that eventually need replenishing.

That’s why it’s important to watch for these signs to determine when plants are ready to move:

Size: Once the plant has grown wider and about twice as tall as the pot it will be struggling to get enough water. You may notice that the soil is drying out more quickly because the plant is taking up the water faster and more moisture will be lost through the larger leaves, particularly if it’s under grow lights.

Soil: After about 8 weeks the plant will have used most of the nutrients in the small pot and growth may slow down.

Roots: Holding the plant upside down with the stem between your fingers you can remove the plant pot and examine the roots. If they are starting to go round and round the edge of the pot then the plant is in danger of becoming ‘pot-bound’ and needs more space.

Leaves: Keep an eye on the plant and note if any leaves start to lose the normal vigorous green colour, curl or droop. Yellowing leaves indicate severe nutrient deficiency and ideally you want to transfer the plant well before this stage.

As soon as one of these signs is noted it’s time to take action:

Transfer the plant outside if conditions are suitable (see below) or

Re-pot the plant in a larger pot with fresh potting soil to give it more nutrients, water and root space. This is a delaying tactic necessary if you can’t transfer the plant when it is ready.

External factors

Unfortunately the weather isn’t always in tune with when our carefully raised plants need to be transferred outside. These are the factors that need to be taken into consideration:

Temperature range: Extremes of temperature are bad for plants that have been used to the careful regulation of light, heat and water. Therefore the best time to transfer plants is not on bright sunny days when the clear skies usually cause the night-time temperature to plummet. It’s better to pick dull overcast days when the temperature range is smaller.

Wind and exposure: Plants raised indoors or under cover won’t have ‘toughened up’ to the air movements outside. Avoid breezy days and exposed areas or put up wind breaks around the plants (as long as they don’t shade them). This year I lost several beans because the wind picked up as I planted them out and the large leaves twisted off the plants in the wind.

Waterlogged, heavy soil: It’s important to delay planting out if the ground is waterlogged, particularly important on clay soils, as this deprives roots of the air they need and can keep the soil temperature too low.

Pests and predators: Early spring often brings many young birds eager for food and they love the tops of pea and brassica seedlings so be aware of this and protect plants if necessary. Likewise, sudden increases of pest numbers such as aphids are often seen in late spring and early summer.

Personal attention: It’s never a good idea to transfer plants when you are just about to go away for a few days. Much better is to choose a time when you will be around to keep an eye on their water levels and whether any pests start to attack them.

I find it’s a good idea to raise more plants than I have room for in the garden and hold some back when planting out. This ensures that if a late frost or unexpected pests damage the plants, I haven’t lost my whole crop. However, it’s important to avoid the mistake of cramming these extra plants into the garden, overcrowding the others!

Transplanting Young Plants

As your plants become ready to make the move outside it is important to get them used to outside temperatures. Over a period of a few days take them outside so they are exposed to the temperature variations and air movements – for details see our article on Hardening Off plants. Take note of the weather forecast to determine the best days for planting out.

Young plants are particularly vulnerable to being eaten so protect from birds if necessary or cover with garden fleece to keep flying insects away. If slugs are a problem, it’s best to put out beer traps a few days before as one slug can munch through a whole row of precious young plants in a night.

When planting out, firm the soil around them and water them well to ensure that the roots are kept moist. Keep a close eye on them in the following days to catch any problems before they develop. When plants are transplanted growth will usually be set back by 1 - 2 weeks as the roots establish themselves after which they will quickly catch up.

Please do add your own tips for helping plants to transfer well to outdoors by adding a comment below...

Comments

"Approx. 60 years ago, a man who had been Head Gardener for the City of Portsmouth said to me: "never plant out your annuals until the lilac is in bloom - you might get a frost but it will be so weak as to be harmless." I've lived by this rule and it's never let me down. This year, the lilac bloomed very early and the first item I planted out was my runner beans - on May 20, they are now nearly 2 metres tall! "doug beardon Saturday 21 May 2011

"I planted out my courgettes, knowing that I was doing it early but the seedlings were growing so fast and they were big. They got put out in all weathers about 2-3 weeks back off the top of my head.
On what was a fine sunny day in the Manchester area, the following day it was due to rain. I could not get there that day but could have waited just one more day.
I planted 9 plants and 3 have survived. What did I learn, it went really cold that night and perhaps I should have waited but I knew I would loose some to slugs, the hot/sudden cold did not help the others.
But they seem to be making a recovery and starting to grow back fine again. The guy next to my plot put his out a week after me, so far they are not doing much but he has not had any losses.
Why? His plot has been in use for a good few years, he has less of a problem with slugs. Mine only been in cultivation for the last 18 months and it is still a work in progress.
Thanks for the tip about the lilac a friend of mine has a lilac bush, will keep a check on its flowering with them and they are only a mile or so from my allotment."Steve Con Saturday 21 May 2011

"We do have lilacs in much of the USA, but many people use apple or dogwood bloom as indicators of "safe" spring. Waiting to plant is sometimes sooo hard, but usually the best course."Barbara Pleasanton Sunday 22 May 2011

"there is no safe anymore with this weather. it is ridiculous to talk long term trends that were good over the last 20-50 years. weather patterns I have watched and used for 30 years are now increasingly defunct. and this is happening with farmers the world over. hello climate change."heatheron Monday 23 May 2011

"There is nothing so satisfying than to get out there in the greenhouse in a world of your own and tend those delicate young plants , watching them grow from seed into healthy strong plants that will produce a good yield of fruit or vegetables "Alan marsdenon Saturday 28 May 2011

"We live in a small valley at 2850 feet in the inland coastal mountains of northern california. Our entire growing season may start as late as June 10, as snow and cold can hang on pretty long sometimes. This year I have planted my starts May 30 with clear plastic cups (bottom cut out) put over each. Learned that trick 40 years ago in Colorado. It keeps them warm, I can cover them with remay and it keeps the bugs off for awhile until they establish."Kate McCay on Tuesday 31 May 2011

"Talking about sowing seeds etc,time has come again to start saving loo and kitchen roll inners to sow seeds in.....What I do is flatten them then fold them in half length ways,open them up and you have a nice square "pot" to sow into.By making them square they fit into seed trays better and when you fill them with compost you don't lose any between the pots like you do when the inners are round.When you need to plant out put the whole lot in the soil the cardboard will rot down so the plant roots will grow out."melboyon Tuesday 20 December 2011

"thanks for the info. This is my first year doing my own seedlings in the green house."irene kastrounison Saturday 10 March 2012

"That's a brilliant idea with the toilet roll inners. Would you then cut them in half or use the full length of the roll at a 'pot' ?
"arthur brogardon Saturday 24 March 2012

" @ arthur you could cut them in half but I always leave them whole when sowing beans.As for kitchen roll inners you could cut them into 3 or 4 depending on what you are sowing...Good Luck."melboyon Monday 26 March 2012

"whoah this weblog is wonderful i love studying your posts.
Stay up the good work! You know, lots of persons are looking around for this
information, you could help them greatly."Romoon Friday 6 September 2013

"my Dad always said don't plant outside until after the first full moon after May 24th"JustLindaon Wednesday 13 May 2015

"Brilliant article!! Very useful for my little seedlings! This has really helped me know when I should start moving them out. Thanks!"Urban Bee and Beeon Saturday 2 April 2016

" When pecan trees bud out it is usually safe to set out your plants. Pecans hate cold weather!"Corliss Pruitton Monday 25 March 2019

" The weather is so unpredictable in Cleveland Ohio. I found that if I take any kind of milk jug huge vinegar jugs and cut the bottoms off. You could transplant your seedlings out into your garden area and put this over of them. You can still water from the top or around the sides but if the wind hits them, or a sudden frost should happen, your little seedlings will survive "Bonnieon Tuesday 2 April 2019

"I have learned the hard way, to wait (at least) 2 weeks after the last expected frost date to plant out my warm weather veggies like tomato and pepper. Even if there was no inclimate weather before the last frost date, any warm weather plants that I had put out never grew any better or faster than the plants I put out later. By mid summer, all the plants were the same size, so no advantage to planting early for me!"Frank on Monday 8 April 2019

"Like the idea about the toilet paper /paper towel roll. They also make good stem protectors for tomato plants if you have slugs or caterpillars eating the stems. Paper “Dixie” cups work too!"Frank on Monday 8 April 2019

"Queztion: I was Ill advised to start coryopsis seeds very late this season and now they are still tiny. Is it ok to plant them in the garden now? If not, how do I treat them over the winter? This is the first time I ever grew anything from seed."Virginiaon Sunday 8 September 2019

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